sha1_name: refactor interpret_upstream_mark

Now that most of the logic for our local get_upstream_branch
has been pushed into the generic branch_get_upstream, we can
fold the remainder into interpret_upstream_mark.

Furthermore, what remains is generic to any branch-related
"@{foo}" we might add in the future, and there's enough
boilerplate that we'd like to reuse it. Let's parameterize
the two operations (parsing the mark and computing its
value) so that we can reuse this for "@{push}" in the near
future.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -1061,35 +1061,36 @@ static void set_shortened_ref(struct strbuf *buf, const char *ref)
 	free(s);
 }
 
-static const char *get_upstream_branch(const char *branch_buf, int len)
-{
-	char *branch = xstrndup(branch_buf, len);
-	struct branch *upstream = branch_get(*branch ? branch : NULL);
-	struct strbuf err = STRBUF_INIT;
-	const char *ret;
-
-	free(branch);
-
-	ret = branch_get_upstream(upstream, &err);
-	if (!ret)
-		die("%s", err.buf);
-
-	return ret;
-}
-
-static int interpret_upstream_mark(const char *name, int namelen,
-				   int at, struct strbuf *buf)
+static int interpret_branch_mark(const char *name, int namelen,
+				 int at, struct strbuf *buf,
+				 int (*get_mark)(const char *, int),
+				 const char *(*get_data)(struct branch *,
+							 struct strbuf *))
 {
 	int len;
+	struct branch *branch;
+	struct strbuf err = STRBUF_INIT;
+	const char *value;
 
-	len = upstream_mark(name + at, namelen - at);
+	len = get_mark(name + at, namelen - at);
 	if (!len)
 		return -1;
 
 	if (memchr(name, ':', at))
 		return -1;
 
-	set_shortened_ref(buf, get_upstream_branch(name, at));
+	if (at) {
+		char *name_str = xmemdupz(name, at);
+		branch = branch_get(name_str);
+		free(name_str);
+	} else
+		branch = branch_get(NULL);
+
+	value = get_data(branch, &err);
+	if (!value)
+		die("%s", err.buf);
+
+	set_shortened_ref(buf, value);
 	return len + at;
 }
 
@@ -1140,7 +1141,8 @@ int interpret_branch_name(const char *name, int namelen, struct strbuf *buf)
 		if (len > 0)
 			return reinterpret(name, namelen, len, buf);
 
-		len = interpret_upstream_mark(name, namelen, at - name, buf);
+		len = interpret_branch_mark(name, namelen, at - name, buf,
+					    upstream_mark, branch_get_upstream);
 		if (len > 0)
 			return len;
 	}